Fallback to yfinance if encountering API error with pandas-datareader.

Update README.md

Fix typo.

Clean up switch to pd.concat().

Add stale action.

Update ticker and examples in README.

Fix column selection in fill_between_other()

Switch append() to pd.concat()

Address compatability issue with pandas > 1.2.0

Update message when raising ValueError in validate_df()

Tweak docstring

Improve docstring

Fix if condition

Fix typos in docstrings

Remove unnecessary ()